1. Field of the Invention
The invention relates generally to activation control apparatus and method of an air bag system of a motor vehicle, and in particular to such an air-bag activation control apparatus that is arranged to activate an air bag device in an appropriate situation so as to protect an occupant upon collision of the vehicle with an object.
2. Description of Related Art
One type of an activation control apparatus of an air bag system as disclosed in Japanese laid-open Patent Publication (Kokai) No. 11-286257 has been known in the art. The activation control apparatus of this type includes a floor sensor that is disposed in a floor tunnel of the vehicle body and is adapted to generate a signal indicative of an impact applied to the vehicle floor portion at which the sensor is located. When a parameter determined based on the output signal of the floor sensor exceeds a threshold value, the activation control apparatus operates to activate an air bag device so as to deploy or inflate an air bag. The apparatus further includes satellite sensors that are disposed in a front portion of the vehicle body and is adapted to generate signals indicative of an impact that is applied to the vehicle front portion in which the sensors are located. The above-indicated threshold value is reduced by an amount that increases with an increase in the impact that is applied to the front portion of the vehicle body and is determined based on the output signals of the satellite sensors. With the threshold value thus reduced, the air bag is made more likely to deploy as the magnitude of the impact applied to the front portion of the vehicle body increases. With the above-described known apparatus, therefore, the air bag device for protecting a vehicle occupant can be activated at an appropriate time or in an appropriate situation.
In the known air-bag activation control apparatus as described above, the amount of reduction of the threshold value serving as a criterion for deployment of the air bag is increased with an increase in the magnitude of an impact that is applied to the vehicle front portion and is determined based on the output signals of the satellite sensors. If the reduction of the threshold value to a desired value is accomplished in one step with no regard to a difference between the threshold value and the desired value, namely, with no regard to the magnitude of the impact applied to the vehicle front portion, the likelihood that the air bag will deploy increases by a large degree at a time. This may happen even in the case where it is determined that a great impact is applied to the vehicle front portion, actually because of an abnormality or defect in the satellite sensors, or the like. In such a case, the air bag may deploy by mistake. In order to cause the air bag to deploy at an appropriate time, therefore, it is considered inappropriate or undesirable to reduce the threshold value to the desired value in one step or at a time. In the known apparatus, however, no special measure has been taken for switching or reducing the threshold value to a desired value without causing erroneous deployment of the air bag.